1. Field of the Invention
The present invention relates to high performance energy conversion devices such as sensors and electromechanical actuators, and, more particularly to energy conversion devices manufactured from boron nitride nanotubes and BNNT/polyimide composite materials.
2. Description of Related Art
Electroactive materials have been studied extensively in the last few decades for use in a variety of applications including electromechanical sensors and actuators, ultrasonic transducers, loudspeakers, sonars, medical devices, prosthetics, artificial muscles, electric energy harvesters and devices for vibration and noise control. Electroactive ceramics such as lead zirconate titanates (PZT), lead-lanthanum zirconate titanate (PLZT), and niobium-lead zirconate titanate (PNZT) have very high piezoelectric coefficients, but have poor mechanical properties (i.e., are brittle) and high toxicity. Compared to the electroactive ceramics, electroactive polymers such as poly(vinylidene fluoride) (PVDF) offer a unique combination of favorable characteristics because they are lightweight, conformable, and tough. However, they have relatively low electroactive coefficients and poor thermal properties.
Recently, a series of amorphous piezoelectric polyimides containing polar functional groups have been developed, using molecular design and computational chemistry, for potential use as sensors in high temperature applications. The piezoelectric response of these polyimides is, however, an order of magnitude smaller than that of poly(vinylidene fluoride) (PVDF). This is due to the fact that the dipoles in the polymer do not align along the applied electric field efficiently because of limited chain mobility within the imidized closed ring structure. To increase the piezoelectric response of these polymers, synthesis with various monomers, control of the poling process, and the adding of carbon nanotubes (CNTs) have been reported.
However, there are still limitations to the use of electroactive polyimide composites in many applications. For example, CNT doped polyimides have large leakage current because the CNTs are either conductors or narrow band gap semiconductors. This limits the use of the composites for high voltage devices. Furthermore, CNTs are chemically active and can be easily oxidized at elevated temperatures (above about 350° C. in air).
Novel electroactive materials have been required for increasing electroactive performance while reducing power consumption for many applications including in the aerospace field. Many electroactive materials have been proposed, but they still have problems of poor mechanical/thermal properties or unsatisfactory electroactive performance. Recently, boron nitride nanotubes (BNNTs) have been successfully synthesized, which exhibit excellent mechanical, electronic, optical, and thermal properties. BNNTs are thought to possess high strength-to-weight ratio, high temperature resistance (about 800° C. in air), and radiation shielding capabilities. Furthermore, intrinsic piezoelectricity of BNNTs has been predicted theoretically. However, no experimental result of the piezoelectric properties of BNNTs or BNNT composites has been reported as yet. In this invention, we demonstrate electroactive actuation characteristics of novel BNNT based materials. We prepared several series of BNNT based electroactive materials including BNNT/polyimide composites and BNNT films. The BNNT based electroactive materials showed high piezoelectric coefficients, d13, about 14.80 pm/V as well as high electrostrictive coefficients, M13, 3.21×10−16 pm2/V2. It is anticipated that the BNNT based electroactive materials will be used for novel electromechanical energy conversion devices.
An object of the present invention is to provide high performance energy conversion devices.
An object of the present invention is to provide high performance energy conversion devices such as sensors.
Another object of the present invention is to provide high performance energy conversion devices such as electromechanical actuators.
Yet another object of the present invention is to provide high performance energy conversion devices manufactured from boron nitride nanotubes and BNNT/polyimide composite materials.
Finally, it is an object of the present invention to accomplish the foregoing objectives in a simple and cost effective manner.